In general, in order to convert a color image into a monochrome image, the brightness of each of the pixels of the monochrome image is determined on the basis of each of the pixels of the color image. For example, according to a technique described in Japanese Laid-open Patent Publication No. 2001-16459, RGB signals in an RGB color space are converted into LHC signals in an LHC space. The converted LHC signals are subjected to color tuning and are re-converted into RGB signals. Thereafter, each of the RGB signals is converted into a monochrome signal Y using the conversion equation “Y=0.3R+0.6G+0.1B” and is output.